The invention is directed to an improved thread stop for a screw jack and a method of forming such a thread stop.
Screw jacks for use with trucks and like vehicles are well known in the art. Such screw jacks must be efficient and safe while relatively low in cost to find acceptance in the modern marketplace. Such screw jacks are generally of the telescoping variety, and include a housing and a screw member carried at least partially within the housing for extensive and retractive or telescoping motion. Generally, a pinion gear is journaled in the housing to turn a crown gear connected to the screw member in order to cause the desired telescoping, jacking motion.
Many advances have heretofore been made in minimizing the cost and complexity of the components of such screw jacks, as well as simplifying the assembly thereof so that screw jacks of this type can be provided at a minimum, competitive cost. However, there remains room for further improvement.
For example, telescoping screw jacks of this variety require some mechanism providing a stop or mechanical limit to the telescoping extension thereof. This is necessary to prevent the jack mechanism from being overextended and inadvertently disassembled while in use. Heretofore, such thread stops have taken various forms. For example in U.S. Pat. No. 1,760,436 to Peteler, a cross pin is provided through a minor thread diameter of the screw member to engage the cooperating, telescoping tubular housing portion, and thereby prevent overextension thereof. Such pins may take the form of set screws or the like set into a sidewall of the screw member. The stop thus fills in a portion of a minor thread diameter to prevent further threadable relative movement between the external threads of the screw member and internal threads of a cooperating telescoping tubular member. It will be recognized that this type of stop requires that the screw member be drilled and tapped to receive the pin or set screw, and a suitable pin or set screw separately provided and inserted properly during the assembly of the screw jack. Hence, care must be taken during assembly of such a screw jack to ensure that these operations are properly carried out. Such extra parts and assembly steps inevitably add to the cost of production of such screw jacks.
The prior art has also proposed the use of a weldment to fill in a part of a minor thread diameter of the screw member in place of such a pin or set screw. However, such a weldment results in an area of relatively harder material in the screw member which may cause wear and damage to the cooperating internal threads of the telescoping tubular member over repeated operations. As an alternative to either a pin or set screw or a weldment, a shearing displacement of the major thread diameter has also been proposed. However, it has been found that the sheared material also presents a relatively abrasive surface to the cooperating internal threads of the telescoping tubular member, which can cause damage in operation. Moreover, the shearing has proven relatively difficult to reliably carry out, since the threads tend to become damaged or even broken during the shearing operation and, in many cases, this results in a non-useable screw member, which must be discarded.